Driven by concerns over global warming caused by greenhouse gases, many attempts have been made to improve the energy efficiency of combustion engines, including spark-ignition and diesel (or more specifically, “compression ignition”) engines, particularly since they are large contributors to many greenhouse gases and other harmful emissions such as carbon monoxide, unburned hydrocarbons, nitrous oxides (NOx) and in the case of diesel engines, particulate matter. Often the approach has focused on emission reductions at the expense of combustion efficiency and fuel economy. For instance, exhaust gas recirculation (“EGR”) has been used, in an attempt to reduce NOx emissions in, e.g., diesel engines. Nitrous oxides form when nitrogen and oxygen are mixed together (e.g., in air), and the mixture is subjected to high temperatures. At high temperatures, N2 and O2 in air disassociate into their atomic states, and a series of chemical reactions result in nitrous oxides. EGR systems introduce cooled exhaust gas into the combustion chamber and reduce NOx via two mechanisms: firstly the exhaust gas is CO2 rich which has a higher heat content and therefore causes the combustion chamber's temperature to be significantly lower; and secondly, the exhaust gas has a lower oxygen content. The lower temperature and reduction in oxygen decrease the NOx formation. However, EGR systems have not provided all the benefits expected as they have been mechanically unreliable and have also resulted in lower overall fuel economy. With economic conditions requiring relatively stable amounts of energy demand per year (e.g. haulage (tons-miles) for freight, electrical power (watt hours) for power generators), the introduction of these “emission technologies” (EGR, and selective catalytic reduction “SCR”) has generally led to lower overall fuel economy and efficiency, and therefore higher greenhouse gas emissions that vary directly with fuel usage (i.e. carbon content in the diesel fuel or heavy fuel oil). These technologies have also increased the cost of engines and the annual operating cost of diesel truck owners or power generators due to higher fuel usage, higher maintenance costs, and higher running costs to support the emissions technologies (e.g. particulate filters, urea for SCR etc.).
The prior art reports numerous attempts to add hydrogen (H2) and/or oxygen (O2) to the pre-combustion mixture to improve combustion efficiency of internal combustion engines. Numerous mechanisms come into play. Hydrogen alone is well known to be an effective fuel with a high caloric value (119.8 MJ/kg versus 42.7 for diesel) and zero carbon residual. The prior art describes the addition of hydrogen and/or oxygen gas to combustion engines to improve fuel economy, including on-board produced hydrogen from the electrolysis of water. The energy required to produce hydrogen from the engine fuel with an “on demand” electrolysis system, however, exceeds the energy content available from the combustion of the hydrogen and there is some controversy about the benefits of such systems. The prior art also suggests, e.g., to combine the added hydrogen with excess oxygen, to produce steam to be subsequently introduced into the combustion chamber to cool the burn at the flame front. This has at times shown reductions in emissions and increased fuel economy.
The inventors believe that a better way to understand the impact of hydrogen is as a diesel or gasoline combustion enhancer. Specifically, hydrogen has a much faster flame speed than, e.g., diesel (2.7 m/s versus 0.3 m/s), higher ignition temperature (585° C. vs. 280° C.), much lower density than carbonaceous fuels and air, and a high diffusivity in air. While the concentration of hydrogen used in this invention is below the lower explosive limit (4% in air), once the diesel ignites, the hydrogen tends to increase the speed of combustion. Likewise the oxygen impacts combustion. Oxygen enriched fuel mixtures tend to burn hotter and faster than standard air mixtures, and addition of oxygen effectively lowers the fuel to air ratio (leaning the fuel to air ratio) and reduces nitrogen in the combustion chamber.
Rhodes et. al. in U.S. Pat. No. 3,262,872 (1966), discloses the use of water electrolysis to produce mixed HHO or Brown's Gas and to inject this mixed gas into the air intake of diesel engines alone with air in trace amounts.
Bari et. al. in Fuel 89 (2010) 378-383; “Effect of H2/O2 Addition in Increasing the Thermal Efficiency of a Diesel Engine” showed that the introduction of relatively small quantities of mixed, stoichiometric H2/O2 (or “Browns” or “HHO” or “hydroxyl”) gas produced in the same ratio obtained from the electrolysis of water, increases the thermal efficiency of diesel engines. Bari demonstrated that the primary mechanism for fuel savings is an increase in thermal efficiency (or “combustion efficiency”), i.e., the percentage of the combustion heat energy that is transmitted into the crank of the engine versus being lost in other forms (e.g. in the heat of exhaust, or in other engine heat losses) is enhanced. Essentially the Browns gas tends to concentrate the combustion in that portion of the cycle where more power can be transmitted to the crank by improving the work (i.e. force times distance) imparted to the piston. As indicated above, this research points to the impact of the hydrogen and oxygen as potential diesel fuel combustion enhancers. However, this work demonstrated that relatively high volumes of H2/O2 gas are required to significantly reduce fuel consumption. Approximately 30 l/min is injected into a 4 l diesel engine to produce fuel economy savings of 15% (i.e., gas input per minute to engine displacement of 7.5 for a 4 stroke engine).
Sheerin in WO 2011/127,583 discloses the addition of non-elemental (i.e. non-stoichiometric) and pre-determined ratios of both hydrogen (H2) and oxygen (O2) to the pre-combustion mixture (i.e., into the air intake) of diesel engines to improve the combustion efficiency with trace additions of gas. The ratio of gases is being held constant while the engine experiences various operating conditions (e.g. loading). These systems work well to improve combustion when load operating conditions are relatively constant and the system can be “tuned” to a narrow operating window.